1. Field of the Invention.
The present invention relates generally to an automatically focusing optical system for copiers, duplicators and other reproduction equipment having a variable magnification capability. More particularly, the present invention relates to an auto-focusing mechanism designed to accommodate imaging lens systems which differ in focal length within the normal manufacturing tolerance band.
2. Description of the Prior Art
The present invention relates to copiers, duplicators and other reproduction equipment with which an original document or the like is to be reproduced. Typically, the optical system of such equipment includes an object plane against which the original document to be reproduced is placed, an image plane against which an imaging medium such as a printing plate or the like is placed, and intermediate optics including one or more mirrors and an imaging lens or lens system for projecting the image of the original document onto the image plane to be reproduced.
When the original and the reproduction are to be the same size or are to vary in size by a constant amount, the various elements in the optical system, i.e., the object plane, mirror(s), lens system and image plane, may be initially located and then maintained in fixed positions relative to one another such that the image projected onto the image plane will always be in focus.
When, however, the system has a variable magnification capability such that the size of the reproduction may be selectively magnified and/or reduced relative to the original, it becomes necessary to change the position of either the object plane, the image plane and/or the mirrors to maintain the system in focus throughout the magnification range. Frequently in office copiers and duplicators, the object plane and the image plane are maintained stationary while the mirror is made movable to maintain proper object to image distance throughout the available range of magnifications, and a variety of systems have been developed in the prior art to accomplish this movement. Usually, these systems incorporate some form of camming means in which the cam surfaces are precisely shaped to effect mirror movement to the positions necessary to achieve proper focus at any magnification.
To ensure exact focus, however, the mirror must be positioned very precisely, and this has resulted in a significant problem. Specifically, it is essentially impossible to make two lenses exactly the same and, as a result, the lenses of different machines will tend to differ slightly in focal length within a normal manufacturing tolerance band (e.g., .+-. 2%). This difference, although seeming to be slight, will affect the focus of the system by a meaningful amount, and, as a result, in different machines, the mirror has to be moved by differing amounts to maintain focus throughout the full range of magnifications. Because of this, it has become the practice in the prior art to maintain a plurality of slightly different cams in inventory and to select the particular cam for use in the machine that is most correct for the focal length of the particular lens system used.
This is obviously an undesirable practice in that it necessitates that a family of cams be maintained in inventory and this increases the cost of manufacturing the machine. Furthermore, the cam finally selected for use in the machine will usually still not be perfectly matched to the optical system but will be only a close approximation of the cam needed.